Did you get a Mystery Code with your copy of Darksiders? Then you are eligible to get a copy of Red Faction: Guerrilla almost for free.
THQ announced that those who still have their code will be getting will be getting a digital copy of the Hellbook disc featuring videos, art and etc. for Darksiders.
Your second item is a free copy of Red Faction: Guerrilla. You get an actual physical copy too rather then digital download, although that means you will need to pay $4.99 for shipping. Both the PlayStation 3 and Xbox 360 version of the game are available.
Unfortunately this deal is US only and ends on March 7th. Be sure to redeem your code before the deal is over.